Connecting The Modbus Rtu (Rs-485) - Siemens SIMATIC NET IE/WSN-PA Link Compact Operating Instructions

Installation and connecting up
3.4 Connector
3.4.3

Connecting the Modbus RTU (RS-485)

Connect the Modbus to the "serial Modbus" connector of the IE/WSN-PA Link (see figure in
section "Connecting the Link (Page 64)").
Modbus RTU
During installation of a serial Modbus connection, the cabling must run from the IE/WSN-PA
Link to the Modbus PLC or the distributed Modbus control system.
The Modbus connectors are located above the center of the terminal block next to the
connector for the power supply. With the Modbus interface, the correct polarity of the
connectors is essential. Connect the minus pin with the Modbus connector (B) at the
extreme right and the plus pin with the central Modbus connector (A). Connector "S" next to
this is for the cable shield (if necessary).
Setting up the Modbus termination
Modbus RTU is transmitted on a physical RS-485 layer. Three DIP switches are used to
create a network terminating resistor for the RS-485 circuit. The switches are located inside
the housing of the electronics on the RS-485 communications board in the center of the
housing at the top.
Figure 3-5
With switch 2 = ON, a 120 ohm resistor is activated on the bus. This is required to match the
cable impedance if reflections need to be damped out when longer cables are used.
Whether or not this terminator is necessary, depends on the transmission speed and the
cable length of the Modbus network.
Switches 1 and 3 = ON are connected to pull-up and pull-down resistors in the Modbus
network. At times when there is no data communication in the network, these resistors
prevent signal noise being interpreted as valid data. In an RS-485 network, only one set of
resistors is required.
Serial RS-485 interface
The RS-485 specifications describe a symmetrical transmission cable in a multipoint
network. Up to 32 nodes can be connected in a network.
66
DIP switches on the Modbus board
Compact Operating Instructions, 12/2012, A5E02509264-AD
IE/WSN-PA Link